1. Field of the Invention
The invention relates to a porous body which has a density of from 40 to 70% of the theoretical density and a predominantly open-pored structure and comprises sintered grains of an Fe-based alloy.
Such porous bodies are used as support substrate in high-temperature fuel cells (solid oxide fuel cells; SOFCs). These are operated at temperatures of from about 650 to 900° C., since only at these temperatures do the thermodynamic conditions for efficient energy generation prevail. In the case of planar SOFC systems, individual electrochemical cells made up of a cathode, a solid electrolyte and an anode are stacked to form a stack and connected by means of metallic components, viz. interconnects, bipolar plates or current collectors. These metallic components have to have specific properties. Thus, the thermal expansion has to match that of the cell materials very well. Furthermore, the metallic components must be highly resistant to corrosion by the anode gas and cathode gas. Corrosion products formed have to have a good electron conductivity. Since interconnects contact anode and cathode, they have the additional function of separating the two gas spaces and therefore have to be completely gastight.
The better the contacting on anode and cathode sides by the interconnect components, the lower the ohmic resistances which are made particularly noticeable by the series connection in the case of planar SOFC systems. To cope with the contacting problems associated with interconnect components better, new planar SOFC designs have been developed, in addition to the application of ceramic, usually perovskite contact slips; in the recent past, the MSC (metal supported cell) has also been developed. Here, for example, porous bodies are laid or welded as support substrates into conventional interconnect components comprising compact material, and the cell materials, usually commencing with the anode layer, are applied directly to these porous bodies by means of coating processes, for example high-speed flame spraying, plasma spraying and spraying of a slurry. The direct connection of electrode and interconnect component brought about in this way thus enables very uniform contacting scaled on the micron scale and also a very uniform gas supply to the electrode to be achieved, with the latter function frequently being performed in conventional planar SOFCs by macroscopic gas channels which have been milled in a complicated process into the surface of dense interconnect components. In addition, the cell materials can be made considerably thinner when porous support substrates are used, since the cell materials are not self-supporting components. This not only allows material to be saved but also makes it possible, for thermodynamic reasons, to reduce the operating temperature of SOFC systems.
The latter advantages of good gas supply and contacting are at the direct expense of disadvantages which can likewise be attributed to the high porosity of the support substrate. Due to the high porosity, the surface area of the support substrate which is in contact with the SOFC-specific gases is very large. This can lead to increased corrosion. In addition, a large surface area also represents a large driving force for sintering processes, as a result of which shrinkage of the porous support plate can occur during operation. The surface area increases with decreasing pore diameter at constant density or with increasing porosity.
For use in MSC (metal supported cell) and ASC (anode supported cell) SOFC systems, it is advantageous to use porous metallic support materials together with conventional interconnect components, since these are cheaper and more ductile than ceramic support materials and also have a higher electronic conductivity. Compared to conventional interconnects, the use of such porous bodies has the advantage that the gas can be supplied through the porous body and that the contact to the cell materials is significantly improved, made more uniform and kept at a constant level during the operating time.
Commercial porous products or those specially developed for SOFC applications, e.g. nonwovens and knitteds, as described in European patent publication EP 1 455 404, international PCT publication WO 02/101859 A2, German patent publication DE 101 61 538 and European patent publication EP 1 318 560, have a satisfactory corrosion resistance and a coefficient of thermal expansion matched to the ceramic cell materials under use conditions which are customary for SOFC systems, i.e. at temperatures of about 650-900° C. in corrosive atmospheres. However, it has been found that application of cell materials or of other ceramic protective layers to these porous support substrates cannot be achieved with sufficiently high quality by means of the above-described coating processes since the porous support substrate made up of fine metal wires/fibers does not offer a uniform attack area and at the same time the mechanical stability is not sufficiently high under use conditions.
German published patent application DE 103 25 862 describes a metallic support substrate having a maximum chromium content of 13%. In the pertinent literature (Werner Schatt, “Pulvermetallurgie Sinter-und Verbundwerkstoffe”, 3rd ed., 1988; p. 371), sintering temperatures of 1100-1250° C. are reported for the production of porous bodies. Since the temperature of SOFC systems in operation extends up to the usual sintering temperature of Fe—Cr materials, commercial porous support substrates produced from compacted, sintered metallic powders tend to undergo after-sintering, so that it is not possible to obtain a porous material having a density of less than 70% of the theoretical density over the long use times. The undesirable after-sintering leads, in particular as a result of the thermocyclic mode of operation of SOFC systems, to irreversible damage to the deposited cell materials. Even the addition of inorganic or organic substances to form pores, as is described in U.S. patent application publication US 2002/0195188 A1 (cf. international PCT publication WO 01/49440), cannot completely prevent after-sintering of an Fe—Cr alloy under the above-mentioned operating conditions since after-sintering is attributable to both surface and volume sintering mechanisms.